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Sorry Einstein, the universe needs quantum uncertainty

By Jessica Griggs

It doesn’t look like quantum mechanics will ever be intuitive. Take away one of its weirdest components – the uncertainty principle – and you end up with a perpetual motion machine. The finding dashes the hopes of those searching for a less bizarre way to make sense of quantum theory, which has made many physicists uncomfortable, including Albert Einstein.

Take away the uncertainty principle and you end up with a perpetual motion machine

Despite being one of the most successful theories of all times in terms of its ability to explain and predict various phenomena, the world of quantum mechanics incorporates many counter-intuitive ideas.

One of these is the uncertainty principle, which states that in the quantum world it is impossible to simultaneously know two quantities, such as a particle’s location and its momentum, with complete accuracy. The more you know about one, the less you can know about the other.

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This is demonstrated in a variation of the famous double slit experiment, where single photons seem to pass through two slits in a barrier at once, producing an interference pattern on a screen that is due to its momentum. Adding a photon detector at either one of the slits, however, removes the uncertainty in the photon’s location, causing the momentum to be more uncertain. This destroys the pattern on the screen.

Nevertheless, accepting that uncertainty exists not due to a lack of knowledge but thanks to a fundamental law doesn’t sit well with some physicists. One famous dissenter was Einstein, who was contemptuous of the idea that unpredictability could be integral to the physical laws governing the universe. Oscar Dahlsten, a theoretical physicist from the University of Oxford, says that a lot of people still intuitively feel that the uncertainty principle should not hold up.

Now Stephanie Wehner and Esther Hänggi at the National University of Singapore’s Centre for Quantum Technology have taken a new tack, recasting the uncertainty principle in the language of information theory.

First, they suggest that the two properties of a single object that cannot be known simultaneously can be thought of as two streams of information encoded in the same particle. In the same way that you can’t know a particle’s momentum and location to an arbitrarily high level of accuracy, you also can’t completely decode both of these messages. If you figure out how to read message 1 more accurately, then your ability to decrypt message 2 becomes more limited.

Next the pair calculate what happens if they loosen the limits of the uncertainty principle in this scenario, allowing the messages to be better decoded and letting you access information that you wouldn’t have had when the uncertainty principle was in force.

Wehner and Hänggi conclude that this is the same as getting more useful energy, or work, out of a system than is put in, which is forbidden by the second law of thermodynamics. That is because both energy and information are needed to extract work from a system.

To understand why, imagine trying to drive a piston using a container full of heated gas. If you don’t know in which direction the gas particles are moving, you may angle the piston wrongly and get no useful work out of the system. But if you do know which way they are moving, you will be able to angle the piston so that the moving particles drive it. You will have converted the heat into useful work in the second scenario, even though the same amount of energy is available as in the first scenario.

Being able to decode both of the messages in Wehner and Hänggi’s imaginary particle suddenly gives you more information. As demonstrated by the piston, this means you have the potential to do more work. But this extra work comes for free so is the same as creating a perpetual motion machine, which is forbidden by thermodynamics (arxiv.org/abs/1205.6894v1).

“The second law of thermodynamics is something which we see everywhere and basically no one is questioning,” says Mario Berta, a theoretical physicist from the Swiss Federal Institute of Technology in Zurich, who was not involved in the work. “Now we know that without an uncertainty principle we could break the second law.”

Since this would be far weirder than the existence of such a principle in the first place, it essentially justifies the uncertainty principle. “This work is about understanding why exactly quantum theory is as it is,” says Berta.

When this article was first posted, its first sentence read “SORRY Einstein, but it doesn’t look like quantum mechanics will ever be intuitive. “